Currently, the International Telecommunication Union-Radio communication sector (ITU-R) is developing a vision of various convergence services based on a 5G network. Also, the South Korean government established a development strategy for the future mobile communication industry in January 2014, and selected a future social network service (SNS), mobile three-dimensional (3D) imaging service, intelligent service, super high-speed service, and ultra high-definition (UHD) imaging/hologram service as five key services.
In addition to this, the European Union, China, Japan, South Korea, etc. established a task force for discussing a 5G network and service, and are discussing a user-oriented 5G service reflecting the lifestyle of 2020, which is the target time of commercializing 5G, in tandem with an innovation of mobile communication technology for providing an ultra high transmission rate of gigabytes per second to users.
Internationally, 5G requirements and technology standards have not yet been determined, but the requirements are expected to be determined for about five different aspects.                Ultra high speed & low latency: 1000 times the speed of Long Term Evolution (LTE), a ultra-low latency response time of less than a few milliseconds, and realistic content        Massive/seamless connectivity: accommodation of 1000 times as many devices and traffic, and ensuring seamless connectivity        Intelligent/flexible network: provision of a software-based structure, real-time data analysis, and intelligent/personalized services        Reliable/secure operation: network availability/reliability of equal to or more than 99%, and self-healing/reconfiguration        Energy/cost-efficient infrastructure: 50 to 100 times the energy efficiency of LTE, and a reduction in the cost of infrastructure/devices        
In 5G mobile communication, for high-capacity transmission, a study is being conducted on the use of a millimeter wave band in which it is easy to ensure a continuous wide bandwidth of a minimum of 500 MHz or more, for example, extremely high frequency (EHF) bands of 27 to 29 GHz and 70 to 80 GHz, but an agreement has not yet been reached. In these EHF bands, it is possible to highly increase the density of antennas. Therefore, when the physical size of an antenna is determined, the interval between radiators constituting an antenna is reduced with an increase of a frequency, and thus an increased number of radiators can be included.
A plurality of radiators serve as the hardware basis of 3D beamforming technology for generating antenna beams in various shapes by controlling the magnitude and the phase of an RF signal and massive multiple-input multiple-output (MIMO) technology which enables multiple transmissions. In this way, it is expected that the 3D beamforming technology for configuring an optimal RF environment and performing high-speed transmission by controlling electric field strength vertically or horizontally according to the distribution of users or by forming several beams and beam switching/tracking technology for providing an optimal link by selecting an optimal beam from among several beams or by changing the beam direction of an antenna according to the location of a user will be actively applied to 5G mobile communication.
FIG. 1 is a diagram showing a configuration of a base station that is applicable to 5G mobile communication. As shown in FIG. 1, a mobile communication cell managed by one base station can be divided into three sectors A, B, and C. Each sector can be divided into a plurality of, for example, 16, beam spots, and RF modules, which have beam antennas to process an analog signal, which can be configured to correspond to the beam spots, on a one-to-one basis.
FIG. 2 shows a connection relationship between media access control (MAC) processing units (simply referred to as “MACs” below) and modems in 5G mobile communication. As shown in FIG. 2, according to 5G mobile communication, a total of 16 beam spots dividing each sector of a mobile communication cell correspond to RF modules which have beam antennas to process an analog signal on a one-to-one basis. The RF modules correspond to modems which perform baseband signal processing, for example, channel coding/decoding, digital modulation/demodulation, multi-antenna processing, and generation of an orthogonal frequency division multiplexing (OFDM) signal, on a one-to-one basis, and the modems also can correspond to MACs which perform mapping between logical channels and transmission channels, error correction, and distribution of time and frequency resources to a plurality of pieces of UE on a one-to-one basis.
Assuming that the physical devices which correspond to the beam spots on a one-to-one basis also correspond to the MACs on a one-to-one basis as indicated by dotted lines in FIG. 2, even when there are little or no UE at a specific beam spot, the corresponding physical layer device needs to be kept turned on at all times. Therefore, energy efficiency is low, and it is difficult to meet the aforementioned 5G requirements.
This work was supported by the Giga KOREA project of MSIP/Giga KOREA Foundation, Republic of Korea. [GK14N0100, Development of millimeter wave-based 5G mobile communication system]